It's tough because it's a not-that problem. It's not this, it's not that, this isn't the cause, that's not the cause.... Since there is no way of knowing the cause or causes, it falls within a gray area in the medical community. Since no one can say why, the government tends to lean towards it doesn't exist.
I can tell you that with lots and lots of testing, most which comes back negative, a doctor might be able to treat a symptom a little bit, when something comes back positive. For instance, my hubby has the ugly sister to Fibro - CFS. He still has it, but since he's a vet, so goes through the VA for health issues (and they're a government administration that refuses to diagnose it, while the CDC does say it's real), his symptoms have been alleviated, a little, through finding out he also has sleep apnea, diabetes, high blood pressure, and a host of other things wrong, that contributed to the fatigue part. The results of all the testing he has endured (and testing for stuff like nueropathy is painful as heII), he is now "only" utterly exhausted, instead of completely exhausted all the time. He can read for 15 minutes, where he used to lose comprehension on the first sentence. He mutters in pain when the famous "flu-like symptoms" hit, instead of groaning loudly. And, on a good day, he can sleep for 10 hours before waking up exhausted, instead of sleeping 8 hours to wake up exhausted.
He, too, is on Disability, but it wasn't any of the health problems or diagnoses given that got him on it. It's tough to fight a ruling when rulings are made by legal definitions and anyone can claim they have something that can't be diagnosed except to say "it's not anything else."